encode: fix encoder_check for B-frames
encode: fix encoder_check for B-frames

xiph/daala,HeadhunterXamd/daala,KyleSiefring/daala,xiphmont/daala,jmvalin/daala,xiphmont/daala,jmvalin/daala,ycho/daala,xiph/daala,HeadhunterXamd/daala,xiphmont/daala,HeadhunterXamd/daala,HeadhunterXamd/daala,ycho/daala,HeadhunterXamd/daala,KyleSiefring/daala,KyleSiefring/daala,luctrudeau/daala,jmvalin/daala,luctrudeau/daala,KyleSiefring/daala,xiphmont/daala,ycho/daala,xiph/daala,xiph/daala,luctrudeau/daala,xiphmont/daala,KyleSiefring/daala,xiph/daala,jmvalin/daala,jmvalin/daala,luctrudeau/daala,ycho/daala,luctrudeau/daala,ycho/daala